This invention relates generally to an arrangement for remotely detecting atmospheric conditions conducive to hazardous ice formation on the exterior of aircraft when in flight. More particularly, this invention relates to an airborne weather radar system having the capability for remote detection of icing conditions. Still more particularly, this invention relates to an airborne weather radar system having increased sensitivity and cooperatively arranged with an air temperature sensor for detecting super-cooled liquid water (SLW) which causes hazardous ice formation.
A substantial problem exists for aircraft flying under conditions conducive to ice formation (icing) on the exterior of the craft. Icing effects on aircraft include decreased lift, decreased thrust, increased drag, increased weight, and increased stalling speed, all of which can result in icing related accidents and, indeed, fatalities. At present there is no airborne equipment available to remotely detect icing conditions. Pilots rely on weather briefings, aircraft de-icing equipment on some aircraft, and pilot training to analyze weather conditions and develop techniques to evade icing conditions once encountered. Unfortunately, weather conditions change rapidly and many icing conditions are so localized that weather briefings are of little significance. Many light aircraft have inadequate or no de-icing equipment. Additionally, these aircraft generally do not have the power to stay airborne when covered with ice. Finally, pilot training is generally less than desireable so that ice build-up conditions occur before being realized by the pilot.
Generally, icing conditions are defined as those conditions where there is visible moisture and the temperature of moisture droplets, or the temperature of the aircraft's surface, is at or below freezing. This definition is not very helpful unless moisture and low temperature conditions can be verified remotely. SLW droplet size is generally in the area of 10 to 200 micrometers in diameter. These small droplets are not detectable by presently available airborne weather radar systems which are designed with a minimum threshold that will detect droplets that are typically 10 millimeters in diameter with rainfall rates starting at 0.7 millimeters per hour. This is in consideration of the fact that airborne weather radar systems operate on the premise that large rainfall rates are associated with hazardous turbulence. Therefore, the system design is optimized for such higher rainfall rates. The icing hazard, on the other hand, is caused by cloud moisture at negligible or zero rate of precipitation which is below freezing temperatures.
U.S. Pat. No. 5,028,929 which issued to Sand and Kropfli on Jul. 2, 1991 relates to a dual
frequency icing hazard detection radar for aircraft. The arrangement covered by the patent compares the attenuation characteristics of SLW at two diverse microwave frequencies, i.e. using two radar systems. The difference in attenuation is proportional to the SLW content. This knowledge, together with the knowledge of the outside aircraft temperature, provides the ability to detect icing conditions. There are several problems associated with this approach: (1) the two radar systems must be calibrated very accurately in terms of sensitivities relative to each other and in terms of their illumination size and location in space; (2) short path lengths within the icing conditions causes errors in the SLW measurement; (3) the SLW droplet sizes must conform to the Rayleigh scattering criteria (they must not be too large) or errors are created; and (4) the hardware is complex and costly, especially for light aircraft which need the most protection against icing conditions.
The present inventor is aware of the following references relating generally to ice formation on aircraft: "The Estimation of Cloud Parameters By Radar", by D. Atlas, Journal of Meteorology, 11, pp. 109-317, 1954; "Statistical Study of Aircraft Icing Accidents" by J. Cole and W. Sand, American Institute of Aeronautics and Astronautics, 10 pp., 1990; "A Flight Investigation of the Meteorological Conditions Conducive to the Formation of Ice on Airplanes" by W. Lewis, NACA TN 1393, 50 pp., 1947; and "Radar Reflectivity of Cumulus Clouds" by H. Sauvageot and J. Omar, Journal of Atmospheric Oceanic Technology, 4, pp. 264-272, 1987.